hdu1272 小希的迷宫 基础并查集

 #include <iostream>
 #include <cstdlib>
 #include <cstdio>
 #include <algorithm>
 using namespace std;

 const int M = ;
 int a, b;
 int father[M];       //记录父节点
 bool circle;         //判断是否存在环
 bool visit[M];       //用来记录顶点数
 int edgenum, vnum;    //分别表示边数，顶点数 

 void initial()
 {
     for (int i = ; i<M; i++)
         father[i] = i, visit[i] = false;
     circle = false;
     edgenum = vnum = ;
 }

 //(查)
 int find(int x)
 {
     return x == father[x] ? x : father[x] = find(father[x]);     //找祖先节点 + 路径压缩
 }

 //(并)
 void merge(int a, int b)
 {
     if (a == b)
         circle = true;
     int x, y;
     x = find(a);
     y = find(b);
     if (x != y)
     {
         father[x] = y;
         edgenum++;       //引出一条边
     }
     else
         circle = true;   //x==y，说明他们是同一个祖先，一旦连通便与祖先3者成环
 }

 int main()
 {
     while (true)
     {
         initial();
         scanf("%d%d", &a, &b);
         if (a ==  && b == )
         {     //为空树
             printf("Yes\n");
             continue;
         }
         if (a == - && b == -)
             break;
         visit[a] = true;
         visit[b] = true;
         merge(a, b);
         while (true)
         {
             scanf("%d%d", &a, &b);
             if (a ==  && b == )
                 break;
             visit[a] = true;
             visit[b] = true;
             merge(a, b);
         }
         for (int i = ; i < M; i++)
         {
             if (visit[i])
                 vnum++;
         }
         if (!circle && edgenum +  == vnum)
             printf("Yes\n");
         else
             printf("No\n");
     }
     return ;
 }